1. Field of the Invention
The present invention relates to a solid state image sensing device including pixels that produce electric signals corresponding to incident light. In particular, the present invention relates to a solid state image sensing device including pixels each of which consists of transistors.
2. Description of the Prior Art
Solid state image sensing devices that are used for various applications can be classified into two main types according to a method to read out/take out photo-charges that are generated in photoelectric conversion elements: CCD type of solid state image sensing devices and CMOS type of solid state image sensing devices: The CCD type of solid state image sensing devices transfer the photo-charges while storing them in potential wells, thereby making a dynamic range thereof narrow, which is disadvantageous to the CCD type of solid state image sensing devices. On the other hand, the CMOS type of solid state image sensing devices directly read out the photo-charges, which are stored in a “pn” junction capacitance of a photodiode, by way of an MOS transistor.
In addition, one of conventional CMOS types of solid state image sensing devices performs a logarithmic conversion operation which converts an amount of incident light by a logarithmic conversion. (See the U.S. Pat. No. 2,836,147.) This solid state image sensing device has a wide dynamic range having five to six digits. Therefore, although an image of a subject having a luminance distribution of a slightly wide range of luminance is sensed, the solid state image sensing device can convert entire luminance information in the luminance distribution into an electric signal so as to be outputted. However, since a region where image sensing is possible becomes larger than the luminance distribution of a subject of an image sensing, there may be generated such an area as has no luminance data in a low luminance range or a high luminance range within the region where the image sensing is possible.
The present applicant has disclosed such a CMOS type of solid state image sensing device as can switch over between a logarithmic conversion operation and a linear conversion operation that are mentioned above. (See the U.S. Pat. No. 3,664,035.) In addition, in order to automatically switch over between the linear conversion operation and the logarithmic conversion operation, the present applicant has disclosed such a CMOS type of solid state image sensing device as sets a potential state of a transistor being connected to a photodiode performing a photoelectric conversion operation to be in an appropriate state. (See the Japanese Patent Application No. 2002-300476.) A solid state image sensing device in accordance with the Japanese Patent Application No. 2002-300476 can change over an inflection point, at which the photoelectric conversion operation is switched over from a linear conversion operation to a logarithmic conversion operation, by changing a potential state of a transistor.
Additionally, in order to reduce dark currents of a solid state image sensing device, such a solid state image sensing device is disclosed as employs an embedded photodiode. (See the Japanese Patent Application 2006-050544.) A solid state image sensing device in accordance with the Japanese Patent Application 2006-050544 comprises an embedded photodiode PD that serves as a photosensitive element; an MOS transistor T1 that has a source thereof connected to a cathode of the embedded photodiode PD; an MOS transistor T2 that has a source thereof connected to a drain of the MOS transistor T1: an MOS transistor T3 that has a gate thereof connected to a connection node between the drain of the MOS transistor T1 and the source of the MOS transistor T2; and an MOS transistor T4 that has a drain thereof connected to the source of the MOS transistor T3.
Then, a direct current voltage VPS is applied to an anode of the photodiode PD and back gates of the MOS transistors T1 through T4, and direct current voltages VRS and VPD are applied to the drains of the MOS transistors T2 and T3, respectively. In addition, signals φTX, φRS and φV are supplied to the gates of the MOS transistors T1, T2 and T4, respectively, and an output signal line 14 is connected to the source of the MOS transistor T4. Moreover, the MOS transistors T1 through T4 are N-channel MOS transistors.
As shown in FIG. 19, a pixel being constructed as described hereinabove comprises an embedded photodiode PD that is constructed by forming a P type layer 20 on a surface of a P type well layer 31 being formed on a P-type substrate 30 so as to have an N type embedded layer 21 buried therein; a transfer gate TG that is provided with a gate electrode 23 being constructed by way of an insulation film 22 on a surface of a region neighboring a region where the embedded photodiode PD is constructed; an N type floating diffusion layer FD that is formed in a region neighboring a region where the transfer gate TG is constructed; a reset gate RG that is provided with a gate electrode 25 being constructed by way of an insulation film 24 on a surface of a region neighboring the N type floating diffusion layer FD; and an N type diffusion layer D that is formed in a region neighboring a region where the reset gate RG is constructed.
In the above-mentioned condition, a P type layer 20 of high density is formed on a surface of the N type embedded layer 21 in the embedded photodiode PD. In addition, the MOS transistor T1 comprises an N type embedded layer 21, an N type floating diffusion layer FD, and a transfer gate TG; and the MOS transistor T2 comprises an N type floating diffusion layer FD, an N type diffusion layer D, and a transfer gate RG. Then, by having an embedded photodiode PD constructed in a pixel in such a manner as described hereinabove, a potential on a surface of the P type layer 20 is fixed to a potential which is same as a potential of a channel stopper layer that consists of a the P type layer surrounding the embedded photodiode PD. Moreover, the N type floating diffusion layer FD has the gate of the MOS transistor T3 connected thereto.
By having a configuration of a circumference of the embedded photodiode PD constructed so as to achieve a configuration as shown in FIG. 19, it is possible to suppress dark currents that are generated on a surface of the circumference of the embedded photodiode PD and to reduce dark currents that are generated in a pixel. In addition, in a signal output circuit which is installed in a subsequent stage of a pixel, a correlation double sampling method can be employed, and thereby, noises can be eliminated. Due to the above-mentioned effects, a solid state image sensing device employing an embedded photodiode PD is expected to be a dominant solid state image sensing device of low noise and high sensitivity.
In addition, in a pixel having such components as shown in FIG. 19, by having a gate voltage of a gate electrode 23 determining a potential state in a transfer gate TG become a midpoint potential, it is possible to switch over an operation between a linear conversion operation that changes an electric signal in a linear manner for an amount of incident light and a logarithmic conversion operation that changes an electric signal in a logarithmic manner for an amount of incident light. FIG. 20A shows a relation of potentials among an embedded photodiode PD, a transfer gate TG and an N type floating diffusion layer FD in a pixel in the above-mentioned condition.
When a light falls on the embedded photodiode PD, a photo-charge is generated. Therefore, the potential of the embedded photodiode PD decreases in accordance with photo-charges that are generated. At this time, when a luminance of a subject of image sensing is low, the potential appearing to the embedded photodiode PD becomes proportional to an integration value of the amount of incident light in a linear manner. Additionally, when a luminance of a subject of image sensing is high, the potential of the embedded photodiode PD becomes low; and when a difference thereof from the potential of the transfer gate TG comes close to a threshold value, the control gate TG works in a sub-threshold region, whereby an electric current flows from the embedded photodiode PD. Then, as shown in FIG. 20A, the potential appearing to the embedded photodiode PD varies so as to be proportional to a logarithmic value of an electric current that is generated by a photoelectric conversion.
When the potential of the embedded photodiode PD varies in accordance with the amount of incident light as described hereinabove, by setting a gate voltage of the gate electrode 23 to be at a low level, the potential of the transfer gate TG comes to take a low value as shown in FIG. 20B, and thereby, the potential of the embedded photodiode PD is maintained as shown in FIG. 20B. After that, a voltage of the potential of the embedded photodiode PD being maintained is transferred to the N type floating diffusion layer FD by way of the transfer gate TG, and additionally, an electric signal due to the voltage being transferred is outputted as an image signal.
However, in a solid state image sensing device including a pixel having a configuration as shown in FIG. 18, in a case where an image signal is produced by having a linear conversion operation performed, a photo-charge that is generated in the photodiode PD by the incident light is stored, and an image signal that is integrated is outputted even though an integrating circuit is not provided therein. On the other hand, in a case where an image signal is produced by having a logarithmic conversion operation performed, an image signal corresponding to a value at a moment when the MOS transistor T1 is turned off is outputted, in spite of a change in the amount of incident light during a period of exposure. As described hereinabove, an image signal that is integrated and converted in a linear manner or an image signal that is converted in a logarithmic manner without being integrated is outputted from a pixel having a configuration shown in FIG. 18. Therefore, compared with a signal at the time of a linear conversion operation having an integrating constituent, a degree of variability of a signal is high, and it is easy to receive effects of noises.
In consequence, at the time of a long-time exposure and the time of a speed light photography when a change in the incident light is likely to occur, there arises a problem that an information on a subject of image sensing cannot be obtained correctly in a case where an image signal being converted in a logarithmic manner is produced, like a case where the luminance of a subject of image sensing is high. In addition, in a case where the image signal being converted in a logarithmic manner is produced, there arises a problem that a flicker occurs for a luminance fluctuation of an illumination.